The role of basal-plane oxygen atoms in determining the ferroelastic and microstructural properties of Y-Ba-Cu-O

Abstract

The basal-plane oxygen atoms in YBa2Cu3O7−x behave like a lattice gas, with very high diffusivity, especially for oxygen-deficient specimens. Implicit in this behaviour is the property that even a small amount of stress applied along theb-axis (b >a) can make these oxygen atoms hop from the (0,1/2,0) sites to the (1/2,0,0) sites. This is suggested as the primary mechanism responsible for the ferroelastic switching observed in this crystal. Since the material is an oxide of a mixed-valence element (Cu), the common occurrence of overall nonstoichiometry is only to be expected. Also, as discussed by Khachaturyan and others, except at very high temperature, the oxygen atoms and the vacancies will always have a tendency for ordering and/or precipitation into configurations which approach near-perfect stoichiometry locally. However, not all evidence for ordering is in conformity with the predictions of Khachaturyan’s concentration-wave model. The experimental data are examined critically. Further experiments to resolve the discrepancies are suggested.